Conversion of hydrocarbons



Patented May 9, 193

UNETD StTTES PATET OFFICE CONVERSION OF HYDROCARBONS No Drawing. Application September 30, 1937, Serial No. 166,585

4 Claims.

This invention relates particularly to the conversion of straight chain hydrocarbons into closed chain or cyclic hydrocarbons.

More specifically it is concerned with a process 5 involving the use of special catalysts and specific conditions of operation in regard to temperature, pressure and time of reaction whereby aliphatic hydrocarbons can be efficiently converted into aromatic hydrocarbons.

In the straight pyrolysis of pure hydrocarbons or hydrocarbon mixtures such as those encountered in fractions from petroleum or other naturally occurring or synthetically produced hydrocarbon mixtures the reactions involved which pro- 5 ,duce aromatics from parafiins and olefins are of an exceedingly complicated character and cannot be very readily controlled.

It is generally recognized in the thermal decomposition of hydrocarbon compounds or hydro- 20 carbon mixtures of relatively narrow range that, Whatever intermediate reactions are involved, there is an overall loss of hydrogen, a tendency to carbon separation and a generally wider boiling range in the total liquid products as compared with the original charge. Under mild cracking conditions involving relatively low temperatures and pressures and short times of exposure to cracking conditions it is possible to some extent to control cracking reactions so that they are lim- 30 ited to primary decompositions and there is a minimum loss of hydrogen and a maximum production of low boiling fractions consisting of compounds representing the fragments of the original high molecular weight compounds.

As the conditions of pyrolysis are increased in severity using higher temperatures and higher times of exposure to pyrolysis conditions, there is a progressive increase in loss of hydrogen and a large amount of secondary reactions involving recombination of primary radicals to form polymers and some cyclization to form naphthenes and aromatics, but the mechanisms involved in these cases are of so complicated a nature that very little positive information has been evolved in spite of the large number of theories proposed. In general, however, it may be said that starting with paraffin hydrocarbons representing the highest degree of saturation that these compounds are,

changed progressively into olefins, naphthenes, 50 aromatics, and finally into carbon and hydrogen and other light fixed gases. It is not intended to infer from this statement-that any particular success has attended the conversion of any given paraflin or other aliphatic hydrocarbon into an 55 aromatic hydrocarbon of the same number of carbon atoms by way of the progressive steps shown. If this is done it is usually withvery low yields which are of very little practical significance.

The search for catalysts to specifically control and accelerate desired conversion reactions among hydrocarbons has been attended with the usual difficulties encountered in finding catalysts for other types of reactions since there are no basic laws or rules for predicting the effectiveness of catalytic materials and the art as a whole is in a more or less empirical state. In using catalysts even in connection with conversion reactions among pure hydrocarbons and particularly in connection with the conversion of the relatively heavy distillates and residue which are available for cracking, there is a general tendency for the decomposition reactions to proceed at a very rapid rate, necessitating the use of extremely short time factors and very accurate control of temperature and pressure to avoid too extensive decomposition. There are further difficulties encountered in maintaining the efiiciency of catalysts employed in pyrolysis since there is usually a rapid deposition of carbonaceous materials on their surfaces and in their. pores.

The foregoing brief review of the art of hydrocarbon pyrolysis is given to furnish a general background for indicating the improvement in such processes which is embodied in the present invention, which may be applied to the treatment of pure parafiin or olefin hydrocarbons, hydrocarbon mixtures containing substantial percentages of paraflin hydrocarbons such as relatively close cut fractions producible by distilling petroleum, and analogous fractions which contain unsaturated as well as saturated straight chain hydrocarbons, such fractions resulting from cracking operations upon the heavier fractions of petroleum.

In onespecific embodiment the present invention comprises the conversion of aliphatic hydrocarbons including paraffin and olefin hydrocarbons into aromatic hydrocarbons by subjecting them at elevated temperatures of the order of Mil-700 C. to contact for definite times of the order of 1 to 10 seconds with catalytic materials comprising major proportions of relatively inert carriers and minor proportions of oxides of vanadium and molybdenum.

In astill more specific and preferred embodiment the catalysts comprise activated aluminum oxide which has received successive and alternate deposits of the two types of oxide. For example, a portion of the vanadium oxide desired in the final composite may first be deposited, then a portion or all of the desired quantity of molybdenum oxides, after which the remaining amount of vanadium oxides is deposited. This procedure may be modified so that molybdenum oxides are first deposited, then vanadium oxides and lastly further amounts of molybdenum oxide or any amount or method of alternation may be employed. The advantages in this method of preparation are definite although the reasons are not fully understood.

According to the present invention aliphatic or straight chain hydrocarbons having six or more carbon atoms in chain arrangement in their structure are catalytically dehydrogenated in such a way that the chain of carbon atoms undergoes ring closure with the production in the simplest case of benzene from n-hexene and in the case of higher molecular weight parafilns of various alkyl derivatives of benzene. Under properly controlled conditions of times of contact, temperature and pressure very high yields of the order of '75 to of the benzene or aromatic compounds are obtainable which are. far in excess of any previously obtained in the art either with or without catalysts. For the sake of illustrating and exemplifying the types of hydrocarbon conversion reactions which are specifically accelerated under the preferred conditions by the present types of catalysts, the following structural equations are introduced.

on, on. on

n-Hemnc benzene CH;\ /C CHz Ca CHr-CH: I OH C a n-Heptane CHK C CHr-CH:

HI CHt-CH! -n-0ctane- In the foregoing table the structural formulas of the primary paraflin hydrocarbons have been represented as a nearly closed ring instead of by the usual linear arrangement for the sake of indicating the possible mechanisms involved. No attempt has been made to indicate the possible intermediate existence of mono-oleflns, diolefins, hexamethylenes or alkylated hexamethylenes which might result from the loss of various amounts of hydrogen. It is not known at the present time whether ring closure occurs at the loss of one hydrogen molecule or whether dehydrogenation of the chain carbons occurs so that the first ring compound formed is an aromatic such as benzene or one of its derivatives. The above three equations are of a relatively simple character indicating generally the type of reactions involved but in the case of n-paraflins or mono-olefins of higher molecular weight than the octane shown and in the case of branched OH ar toluene o-xylene chain compounds which contain various alkyl substituent groups in different positions along the six-carbon atom chain, more complicated reac-' hexane the principal resultant product is apparently o-xylene although there are concurrently produced definite yields of such compounds as ethyl benzene indicating an isomerization of two substituent methyl groups. In the case of nonanes which are represented by the compound 2,3,4-trimethyl hexane, there is formation not only of mesitylene but also of such compounds as methyl ethyl benzol and various propyl benzols.

It will be seen from the foregoing that the scope of the present invention is preferably limited to the treatment of aliphatic hydrocarbons which contain at least 6 carbon atoms in straight chain arrangement. In the case of paraffin hydrocarbons containing less than 6 carbon atoms in linear arrangement, some formation of aromatics may take place due to primary isomerization reactions although obviously the extent of these will vary considerably with the type of compound and the conditions of operation. The process is readily applicable to paraflins from hexane up to dodecane and their corresponding oleflns. With increase in molecular weight beyond this point the percentage of undesirable side reactions tends to increase, and yields of the desired alkylated aromatics decrease in proportion. Beginning with decane and under the more severe conditions of operation with regard to temperature and time of contact with the catalyst, there is some tendency for the formation of polynuclear cyclic compounds such as naphthalene and anthracene which result from too extensive hydrogenation reactions. However, when conditions are properly chosen, the formation of these compounds may be kept at a practical minimum.

The present invention is characterized by the use of a particular group of composite catalytic materials which employ as their base catalysts or supporting materials certain refractory oxides and silicates which in themselves may have some slight specific catalytic ability in the dehydrogenation and cyclization reactions but which are improved greatly in this respect by the addition of vanadium and molybdenum oxides in minor proportions. The base supporting materials are preferably of a rugged and refractory character capable of withstanding the severe use to which the catalysts are put in regard to temperature during service and in regeneration by means of air or other oxidizing gas mixtures after they have become fouled with carbonaceous deposits after a period of service. As examples of materials which may be employed in granular form as supports for the preferred catalytic substances may be mentioned the following:

Montmorillonite clays Kieselguhr It should be emphasized that in the field of catalysis there have been very few rules evolved which will enable the prediction of what materials will catalyze a given reaction. Most of the catalytic work has been done on a purely empirical basis, even though at times certain groups of elements or compounds have been found to be more or less equivalent in accelerating certain types of reactions.

' In regard to the base catalytic materials which are employed as supports according to the present invention, some precautions are necessary to insure that they possess proper physical and chemical characteristics before they receive the pared by precipitation or other chemical methods may be used alternatively in place of the natural mineral. It is not necessary that the carbonate be completely converted to oxide but as a rule it is preferable that the conversion be at least over 90%, that is, so that there is less than 10% of the carbonate remaining in the ignited material.

A very effective catalyst consists of activated aluminum oxide (preferably slightly alkalized) supporting successive depositions of about 2% vanadium sesquioxide V203, 4% of molybdenum sesquioxide M0203, and another 2% of the vanadium compound. The exact method of preparation of this type of catalyst will be described in later paragraphs in greater detail. It is essential to the preparation of these catalysts that the aluminum oxide possess certain structural characteristics permitting the maintenance of a stable deposit of the oxideson its surface which is essentially undisturbed under the conditions of operation and when regenerating by burning oif carbonaceous deposits with air or other oxygen-containing gas mixtures. Aluminum oxide which is generally preferable as a base material for the manufacture of catalysts for the process may be obtained from some natural aluminum oxide minerals or ores such as bauxite orcarbonates such as dawsonite by proper calcinationfor it may be prepared by precipitation of aluminum hydroxide from solutions of aluminum sulfate, nitrate, chloride, or different other salts, and dehydration of the precipitate of aluminum hydroxide by heat. Usually it is desirable and advantageous to further treat it with air or other gases, or by other means to activate it prior to use.

Two hydrated oxides of aluminum occur in nature, to wit, bauxite having the formula A12Oa.2H2O and diaspore having the formula A1203.H20. Of these two minerals only the corresponding oxide from the bauxite is suitable for the manufacture of the present type of catalysts and this material has generally given the best results of any of the supporting materials which have been tried. The mineral dawsonite having the formula Na2Al(C03)a.2Al(0H)3 is another mineral which may be used as a source of aluminum oxide, the calcination of this mineral giving an alkalized aluminum oxide which is apparently more efiective as a support in that the catalyst is more readily regenerated after a period of service. Alumina in the form of powdered corundum is not suitable as a support.

It is best practice in the final steps of preparing aluminum oxide as a base catalyst to ignite it for some time at temperatures within the approximate range of from GOO-700 C. This does not correspond to complete dehydration of the oxide but gives a catalytic material of good strength and porosity so that it is able to resist for a long period of time the deteriorating effects of the service and reactivation periods to which it is subjected.

The oxide of vanadium which results from the ignition of the nitrate, the hydroxide or the carbonate is principally the pentoxide V205 which is reduced by hydrogen at a red heat to form the tetroxide V204 or the corresponding dioxide V02 and then to the sesquioxide V203. The production of the deposits of sesquioxide upon the granular carriers (preferably alumina) may be made by the vanadyl nitrate or solutions of aluminum or alkali metal vanadates, some of which furnish alkaline residues on ignition. The use of ammonium vanadate is preferred.

The two most important oxides of molybdenum which are alternatively employed as catalysts according to the present invention are the dioxide M002 and sesquioxide M0203. Since the reduction of the trioxide by hydrogen begins at 300 0. (572 F.) and the reduction is rapid at 450 C. (842 F.) the effective catalytic material is principally the sesquioxide. The trioxide may be added to the active alumina carrier from a solution in aqueous ammonia or from a solution of ammonium molybdate which are added in amounts just requisite to wet the carrier granules uniformly and the mass is then dried and ignited.

The following example of the preparation of a catalyst comprised within the scope of the invention and its use in dehydrocyclization reactions is given to show the advantages and practicality of the invention although not with the intent of imposing undue limitations thereon.

A granular activated alumina comprising par-' ticles of approximately 4-12 mesh was given successive additions of vanadium sesquioxide, molybdenum sesquioxide, and a further amount of the vanadium compound by first adding a solution of ammonium vanadate of a concentration and-in an amount suflicient to leave a residue of about 2% V203 by weight after reduction of the primary oxides. After drying a solution of ammonium molybdate was then added to produce ultimately a deposit of 4% by weight of M0203 after which the primary step of adding the ammonium vanadate solution was repeated so that ultimately after drying and reducing the composite catalyst had the following approximate composition:

Per cent A1203 92 V203 4 M0203 4 ing pure n-heptane over each at 500 C., atmos- V pheric pressure, and a contact time of about 3 seconds. The following tabulation shows the results obtained:

Oncethrough Catalyst yield of toluene 92% AI203-8% V203 m 92% A1203-8% M0103 21. 92% Altos-4% VzO3-4% IVIOzOs 30 It will be seen from the above data that the activity of the catalyst comprising 92% alumina and supporting 4% each of vanadium and molybdenum sesquioxides was approximately 50% greater than that of the catalysts comprising respectively 92% aluminum oxide and 8% of either vanadium sesquioxide or molybdenum sesquioxide. It was further observed that the rate of carbon formation due to side reactions was much less in the case of the mixed oxide catalyst as compared with the two simpler oxide catalysts. This indicates that there is evidently a double promotion effect so that there is a deflnite advantage gained in using both oxides.

The nature of the present invention as an improvement in catalytic hydrocarbon conversion reactions is evident from the preceding specification and numerical data although neither section is intended to be unduly limiting thereon.

We claim as our invention:

1. A process for the production of aromatic hydrocarbons from aliphatic hydrocarbons of from six to twelve carbon atoms, which comprises dehydrogenating and cyclicizing the aliphatic hydrocarbon by subjection to a temperature of the order of 400 to 700 C. for a period of time not exceeding 10 seconds in the presence of a catalyst comprising vanadium oxide and molybdenum oxide.

2. A process for the production of aromatic hydrocarbons from aliphatic hydrocarbons of from six to twelve carbon atoms, which comprises dehydrogenating and cyclicizing the allphatic hydrocarbon by subjection to a temperature of the order of 400 to 700 C. for a period of time not exceeding 10 seconds in the presence of a catalyst comprising a major proportion of aluminum oxide and minor proportions of vanadium oxide and molybdenum oxide.

3. A process for the production of aromatic hydrocarbons from aliphatic hydrocarbons of from six to twelve carbon atoms, which comprises dehydrogenating and cyclicizing the aliphatic hydrocarbon by subjection to a temperature of the order of 400 to 700 C. for a period of time of about 1 to 10 seconds in the presence of a catalyst comprising vanadium sesquioxide and molybdenum sesquioxide.

4. A process for the production of aromatic hydrocarbons from aliphatic hydrocarbons of from six to twelve carbon atoms, which comprises dehydrogenating and cyclicizing the aliphatic hydrocarbon by subjection to a temperature of the order of 400 to 700 C. for a period of time of about 1 to 10 seconds in the presence of a catalyst comprising a major proportion of activated aluminum oxide and. minor proportions of vanadium sesquioxide and molybdenum sesquioxide;

JACQUE C. MORRELL. ARIS'IID V. GROSSE. 

